This invention relates to injection molding machines using sprues, and more particularly, to a nozzle for use with a sprue which is capable of tolerating positioning error of the sprue while still forming a good seal therewith.
For injection molding applications where there are more than 2 levels in the mold, multiple sprue bars can be used for delivering a split resin stream to the multi-level injection mold. That is, after the resin stream is split, the sprue bars carry the resin to the mold sections comprising the injection mold. With multi-level sprue bar applications, typically a single source channel is used with a nozzle which breaks the single source channel into a plurality of channels aligned with the individual sprue bars. Typically, the sprue bars are attached with the mold section to which the molding resin is being delivered. Because injection molding devices generally move in the longitudinal or vertical direction, the sprue bars must be displaced with the platens. Accordingly, the sprue bars are not rigidly attached to the nozzle or in the channel from which the molding material is received and as such, the sprue bar arrangement must be designed so that the sprue bars will return to the channel splitting nozzle and reform a seal therewith.
In particular, for multi-level stack molds where resin must flow from a single source injection unit to multiple levels spaced progressively farther from the stationery platen, several design problems are typical. For example, in a four level stack mold, a sprue bar will feed the first and second level via channels in the mold plate between the two levels and another sprue bar will feed the third and fourth levels via channels in the mold plate between these two levels. Because of the progressive arrangement, these two sprue bars will necessarily be of different lengths. Additional variations in length occur due to each sprue bar being subject to thermal expansion effects. Accordingly, when the mold is closed, the position of the end of each sprue bar relative to the stationery platen and the channel splitting nozzle and the other sprue bar, will vary due to variations in the position in which the mold plates close between various levels. The combination of these variables makes it almost impossible to predict the location of the two sprue bar ends or bushings each time the mold is closed and the sprue bar is returned to the multiple channel nozzle.
The prior art contains several devices directed toward injection units having single source and multiple channel supplies and corresponding attempts to solve the problems of creating reliable and repeatable seals between the multi-channel nozzle and the sprue bars or similar channels.
U.S. Pat. No. 3,843,295 discloses an injection molding machine with adjustable nozzle length means. The machine comprises a plurality of split molds mounted upon a plurality of platens having parallel faces which reciprocate linearly for the opening and closing of the molds. Each mold cavity is filled through one or more nozzles extending from a fixed manifold portion of the machine. Each nozzle has a tip at least intermittently fitting against a nozzle seat. Length adjustment means are provided for each nozzle. The molding machine is operable at a plurality of temperatures, while still having a reciprocatingly retractable leakproof fit between certain nozzle tips and their nozzle seats. The spring based nozzle in FIG. 4 can accommodate a varying seating location wherein the resulting sealing force will vary according to the spring compression. However, this results in inconsistent performance. FIG. 5 of the same patent shows a hydraulically actuated nozzle which also accommodates variation while maintaining a consistent sealing force. However, the use of hydraulic fluid in close proximity to hot resin is very dangerous since the fluid is flammable and the complexity of the fluid supply conduits increases costs and maintenance.
U.S. Pat. No. 3,915,610 discloses an alignment assembly for plastic injection molds. The alignment assembly includes a base communicating with a respective cavity and counterbore. A valve body is provided which has a stem portion slidably and sealingly disposed in the bore and a head portion disposed in the counterbore. The body has a through portion and the head portion is configured to mate with an ejection nozzle. The sealing fit of the stem allows axial movement of the body to react to the pressure generated in the cavity by the injected plastic and thereby be forced tightly against the nozzle to prevent flash. The apparatus disclosed relies upon the pressure of the plastic pushing the body 32 against the nozzle to maintain the seal between the nozzle and the body. The seal on the stem is created by a tangential contact between the stem and bore. The device requires that the stem pivot and slide in the bore to create and maintain alignment. This will result in wear on the spherical portion which will lead to clearance and eventually leakage. Also, since the contact area is so small, there is little capacity to tolerate wear.
U.S. Pat. No. 4,299,791 discloses a method for preventing the drooling of a plastic injection molds and injection nozzles. The patent discloses a spring biased sprue member used to accommodate a change in resin volume following injection. The device deals with only one nozzle/sprue connection and the spring design does not serve to accommodate varying locations of nozzles through contact.
U.S. Pat. No. 4,917,595 discloses a nozzle touch sprue bushing device. The device is for use with a unit-type injection metal mold which includes a molding body having an inner mold element which is removably mounted on an outer frame of an injection molding machine. The sprue bush has a nozzle touch provided at a rear end thereof for fitting with an end of a nozzle of an injecting machine. The sprue bush also has a front end formed in such a fitting configuration so as to allow fitting engagement with a material admitting portion of the mold. All of the methods disclosed in this patent include spring means used to bias the intermediate conductor away from the inner mold inlet until the nozzle is engaged. The sprue bush is spring biased and substantially rigidly attached and does not allow for angular misalignments of the injection nozzle or the plate bush.
There exists a need, therefore, for a nozzle for use with a sprue which is return position error tolerant and which allows the formation of a tight reliable repeatable seal between the sprue and the nozzle.